The Lymphatics

Abstract

The lymphatic vessels develop secondary to the blood vessels either phylogenically or ontogenically. Amphibians and reptiles possess enormous lymph sacs or sinuses encircling the aorta and almost all other arterial and venous blood vessels in order to store water and prevent loss of heat through use of solar radiation. In birds and mammals, animals which can maintain the body temperature constant, the lymphatics become abruptly narrow and tubular, and tend to run independently from the blood vessels. Anastomoses between blood and lymphatic vessels also decrease. Ontogenically, the main lymphatic vessels and ducts of mammals arise directly by sprouting of venous endothelial cells of both sides of two junctions between the cervical and subclavian veins and between the caudal vena cava and renal vein. The openings at the latter communication site remain rarely as lymphatico-venous anastomoses. The lymphatic vessels are not distributed in all tissues and organs. Neither blood nor lymphatic vessels are present in the epithelium (excepted for stria vascularis of the inner ear), sclera, cartilage, or intima of blood vessels. The central nervous system is rich in blood vessels but does not contain internal lymphatics. The lymphatic capillaries are usually located at a distance from the blood capillaries in order to absorb tissue fluid that becomes filtered from the latter [1]. The neighboring endothelial cells of lymphatic capillaries are separated by a wide gap, which facilitates the passage of erythrocytes. After closure of the gaps, the lymphatic capillaries continue to absorb tissue fluid through intracellular pinocytotic vesicles; this mechanism is facilitated by almost absence of a basal lamina. Furthermore, pericytes are absent; anchoring filaments prevent the collapse of lymphatic capillaries [2]. The role of anchoring filaments as the sole structures reinforcing the lymphatics wall has found a wide acceptance, however, recent works increasingly favor the role of reticular fiber network of the basement lamina as additional wall structures supporting the lymphatic endothelium [13]. The collecting lymphatics and lymphatic ducts possess various amounts of smooth muscle fibers in rich connective tissue, contracting intrinsically. However, the major mode of lymph propulsion is passive, such as that occurring during massage or respiration. The presence of many valves is important for regulation of lymph flow [2, 3, 11].